The present invention relates to a low-temperature storage constituted of an insulating box article having a foamed insulating material and a vacuum insulating panel between an outer box and an inner box, and a method of manufacturing the low-temperature storage, more particularly to a low-temperature storage in which the inside of an inner box is set to an extremely low temperature of, for example, −80° C. or less.
Heretofore, in the insulating box article constituting a low-temperature storage such as a refrigerator or a freezer, a space constituted by combining an inner box and an outer box is filled with a foamed insulating material to reduce a leakage of cold in the inner box. At this time, a temperature difference between the outside and the inside of the inner box is considered in determining a thickness of the insulating material to fill between the inner box and the outer box. However, to keep the temperature in the inner box at an extremely low temperature of, for example, −80° C. or less, a considerable thickness of the insulating material has to be secured. Therefore, to secure a storage amount of a storage chamber of the inner box, the low-temperature storage itself is enlarged, and power consumption is large.
To solve the problem, as a technique for reducing a thickness dimension of the insulating box article, a vacuum insulating material is disposed in a space between the inner box and the outer box, and gaps among them are filled with a foamed insulating material such as urethane foam (see Japanese Patent Application Laid-Open No. 8-68591). This vacuum insulating material is constituted by putting, in a bag-like container constituted of a multilayered film, a gas-permeable bag including an inorganic fine powder and preliminarily formed into a predetermined shape; discharging air from the bag; and then heat-sealing the bag. In consequence, there is obtained an insulating capability greater than an insulating effect obtained by filling the gaps with the usual foamed insulating material, which makes it possible to reduce the thickness dimension of the insulating box article.
However, in the method of disposing the conventional vacuum insulating material in the insulating box article, the above-described temperature in the inner box is set to an extremely low temperature such as −80° C., the cold in the inner box reaches the surface of the bag which covers the vacuum insulating material, and the temperature of the surface of the bag lowers below a heat-resistant temperature of the bag owing to the cold. As a result, thermal contraction occurs, and the bag itself is destroyed. Therefore, the main body of the vacuum insulating material cannot exert its insulating capability. In consequence, the above method cannot maintain the insulating effect.